Metal vapor lamp having a heat reflecting coating of calcium pyrophosphate



3EQ-M1 5R {W mmuHRoom June 13, 1967 c. G. COOK 3,325,662

METAL VAPOR LAMP HAVING A HEAT REFLECTINO COATING OF CALCIUMPYROPHOSPHATE sUBSTH-UTE FQR M\SS1NG XR Filed Sept. 19, 1965 Invefvtor:Clarence .Cook

His A FL vneg I V I i i in 4 X/WXU/IL' United States Patent Q 3,325,662METAL VAPOR LAMP HAVING A HEAT REFLECT- ING COATlNG OF CALCIUMPYROI'HOSPHATE Clarence G. Cook, Mayfield Heights, Ohio, assignor toGeneral Electric Company, a corporation of New York Filed Sept. 19,1963, Ser. No. 309,965 2 Claims. (Cl. 31317) This invention relates tometallic vapor lamps using an arc discharge in mercury and metal halidevapors to produce visible light, and is more particularly concerned withreflective coatings on the ends of the are tube for controlling itstemperature distribution.

The mercury arc lamp has achieved commercial acceptance by virtue of itslong life and reasonably good efficiency but suffers from relativelypoor color rendition due to the bluish-green quality of its light. Alsoits efliciency in the range of 50 to 60 lumens per watt is appreciablybelow the 70 to 80 lumcns per arc watt range of the ordinary fluorescentlamp. A radical improvement in both color rendition and efliciency maybe achieved by adding to the mercury one or more vaporizable metalhalides under proper control of loading, temperature and pressure, thepreferred metal halide additive being sodium iodide, optionally withthallium iodide. Such improved lamps are described and claimed incopcnding application Ser. No. 84,068 of Gilbert l-l. Rolling, filedJan. 23, 1961, entitled Gaseous Electric Discharge Lamps" and assignedto the same assignec as the present invention, now Patent 3,234,421. Forconvenience, such lamps will henceforth be referred to herein as mercurymetal halide lamps.

in its general construction and appearance, the mercury metal halidelamp resembles the conventional highpressure mercury vapor lampcomprising a quartz arc tube mounted within a glass outer jacket havinga screw base at one end. Thermionic main electrodes are provided at theends of the arc tube which contains a quantity of mercury and metalhalide such as sodium iodide along with an inert gas such as argon forstarting purposes.

The mercury vapor lamp is usually designed to operate with its charge ofmercury entirely vaporized, the vapor then being unsaturated. Thisrequires that all parts of the p the electrodes, that is the ends of thearc tube, are the coolest regions in normal operation of thelamp. In theabsence of special measures to raise the temperature of the ends, it isfound that in a mercury metal halide lamp, the metal halide such assodium iodide rapidly condenses on the envelope wall behind theelectrodes, making the lamp ineffective.

In certain sires of mercury vapor lamps where the ends of the are tubetend to operate at too low a temperature, various kinds of reflectivecoatings have been applied to the tube ends n order to raise theirtemperature. However, I have found that metallic reflective coatings,such as coatings of platinum, nickel, stainless steel, gold or silverwhich can be used with mercury vapor lamps where .he interenvelope spaceis filled with an inert or inactive gas, are not suitable for use withmercury metal halide lamps where the intercnvelope space is evacuated asa heat conservation measure. in evacuated jackets, the material of suchreflective coatings vaporizes away from the arc tube, resulting in poorheat shields and simultaneously causing severe blackening anddiscoloration of the jacket.

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Such materials may also give off gases which destroy the effectivenessof the vacuum in the jacket.

The object of the invention is to provide rcflectively coated ends onthe arc tube of a mercury metal halide lamp which are effective forraising the temperature of the ends and which are suitable for use in ahigh vacuum jacket.

Another object is to provide a coating of the recited kind which doesnot release gas, adheres well to the quartz and which is inexpensive andeasy to apply.

Aecording'to my invention, I have found that calcium pyrophosphate, c ro, may be used as a coating on the ends of the arc tube and meets allthe necessary requirements for a heat shield. It is free from gases,adheres well to the quartz tube, has excellent reflectivity and lowcost, and is easy to apply. It is preferably applied as a suspension inwater with a fractional percentage of fine silica to promote adherence.

For further objects and advantages and for a better appreciation of theinvention, attention is now directed to the following detaileddescription of a preferred embodiment to be read in conjunction with theaccompanying drawing. The features of the invention believed to be novelwill be more particularly pointed out in the appended claims.

The single figure of the drawing is a side view of a mercury metalhalide arc lamp embodying the invention.

Referring to the drawing there is shown a high-pressure mercury vaporlamp 1 comprising an outer vitreous envelope or jacket 2 of generallytubular form modified by a central bulbous portion 3. It is provided atits outer end with a re-entrant stem 4 having a press 5 through whichextend relatively stifl' inlead wires 6, 7 connected at their outer endsto the contacts of the usual screw-type base 8, namely the threadedshell 9 and the insulated center contact 10.

The inner arc tube 12 is made of quart or fused silica and has sealedtherein-at opposite ends a pair of main arcing electrodes 13, 14. Theelectrodes have inlcads 15, 16, respectively, each including anintermediate thin foil section 17 hermetically sealed through fulldiameter pinch seals 18, 19 at the ends of the arc tube. Each electrodecomprises a tungsten wire helix wrapped around a tungsten core wire andmay be activated by providing a small elongated piece or sliver ofthorium metal (not shown in the drawing) inserted between the core andthe helix. Alternatively, the electrodes may be activated by a very thinlayer of thorium metal vacuum'deposited thereon. The are tube contains aquantity of mercury which is entirely vaporized during operation of thelamp and which at such time exerts a pressure in the range of 1 to 15atmospheres. A rare inert gas such as argon is provided at a lowpressure, for instance at approximately 25 millimeters of mercury,within the arc tube to facilitate starting and warm-up. In addition aquantity of a metal halide, suitably sodium iodide, is provided inexcess of that vaporized at the operating temperature of the arc tubewhich should be not less than 500 C. at any place.

The are tube is supported within the outer jacket by a frame or harpcomprising a single side rod 21 and rod portions 22 on the open side.The frame is welded at its base end to inlead wire 7 and has a pair oftransverse spring members 23, 24 which against the outer tubular portionof the jacket to provide lateral support. The are tube is fastened tothe frame by a pair of metal straps 25, 26 which extend between rod 21and rod portions 22 and encompass and clamp the pinch seals. The strapsare spaced :1 distance away from the ends of the arc chamber in order.to limit the cooling effect and also to prevent possible devitrificationof the quartz about the electrode inleads. Electrode 13 is connected byconductor 27 a, r a j e threaded through insulating glass sleeve 28 toinlead wire 6 of the outer jacket and thereby to base shell 9. Electrode14 is connected by conductor 29 to rod 21 which in turn is connected byinlcad wire 7 to center contact 10 of the base.

For maximum efliciency, it is desirable to reduce the heat losses fromthe are tube. Therefore, as a heat conservation measure, theinterenvelope space is evacuated prior to scaling off the outer jacket.Getter material is provided in the channelled rings 30'and flashed aftersealing of the jacket in order to assure high vacuum; a suitable getteris barium metal powder pressed into the rings. To prevent condensationof sodium iodide at the ends of the arc tube behind the electrodes thetemperature in these regions during operation should be not less than-500 C. This minimum temperature is assured by applying a heat reflectivecoating, indicated by the speckling, to the ends of the are tube and tothe adjacent portions of the pinch seals. Desirably, the reflectivecoating should extend along the walls of the are tube up to where it isapproximately flush with the tip of the electrode. There is thus formeda concave reflector which is highly effective in preserving heat whileobstructing the visible light gencratcd in the interelectrode path to aminimum extent. In the direction of the outer end, the reflectivecoating should extend over the pinch at least up to the beginning of thefoil section of thc inlead. However the extent is not critical andsuitably the reflective coating may be extended up to the straps whichclamp the arc tube to the frame.

My invention is more particularly concerned with the nature and mannerof application of the reflective coating pyrophosphate. CilzPgOq is asuperior material for a heat at the ends of the arc tube. I have foundthat calcium reflective gttm inasmuch as it meets all the necessary ieqtriren'tents. It is relatively easy to apply, it adheres well to thequartz tube, has excellent reflectivity and is low in cost. Also it doesnot release any gases which would dcleteriously affect the vacuum in theintcrcnvclopc space.

A suitable method of applying the reflective coating is as follows. Asuspension of Cit-W in a water soluble binder is prepared by mixing thematerial in an ammoniacal aqueous solution of the copolymer of polyvinylmethyl ether and maleic anhydride. Suitable proportions are 0.3% to 0.8%by weight of the copolymcr relative to weight of Ca P O To this mixtureis added from 0.1% to 1% of fine silica, suitably /2% by weight relativeto Ca P O This combination of ingredients is milled for approximately 5hours before application to the arc tube. Prior to application, thequartz surface of the are tube must be freed of any grease, dust orother foreign parthe exposed ends are heated by directing the flames ofgas 'burners against them. After the quartz has been preheated, the Ca PO suspension is applied as a spray using a high pressure air gun. Theflames are then again directed on the ends in order to bake out thebinder and any other residue which may have been captured within the CaP O suspension. The preheating of the quartz is important and results inmuch better adherence of the coating.

Life tests of Ca P O coatings applied in the manner described show noevidence of a decrease in reflectance of the coating with time andtemperature. The coating on the ends results in an increase in lightoutput due to better hcat conservation and less light absorption. Themetal halide such as sodium iodide is prevented from condensing behindthe electrodes whereby higher efliciency and better color rendition areachieved.

While the invention has been described by reference to a specificpreferred embodiment, the details of construction described are intendedas exemplary and not in order ticles which might interfere with properadhesion. Scrubhing with alcohol followed by distilled water has provensat sfactory. A mask is then placed over the are tube leaving only theends exposed to which the reflective coating is to be applied. The tubeis held at both ends in a suitable fixture and rotated about its axis ata relatively low speed, for instance revolutions per minute. At the sametime,

to limit the invention thereto except insofar as any may be included inthe accompanying claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge lamp comprising a vitreous outer jacketenclosing a quartz arc tube having a pair of thermionic electrodessealed therein at opposite ends and containing an ionizable fillingcomprising mercury, a metal halide, and an inert starting gas, theinterenvelope space between said jacket and said are tube beingevacuated, and a heat reflective coating on the ends of said are tubeextending approximately flush with the tips of said electrodes andconsisting essentially of calcium pyrophosphate Ca P O 2. An electricdischarge lamp comprising a vitreous outer jacket enclosing a quartz arctube having a pair of thermionic electrodes sealed therein at oppositeends and containing an ionizablc filling comprising mercury, sodiumiodide, and an inert starting gas, frame means supporting said are tubewithin said jacket, the intcrcnvclope space between said jacket and saidare tube being evacuated, and a heat reflective coating on the ends ofsaidare tube extending approximately flush with the tips of saidelectrodes and consisting essentially of calcium yrophosphate CHZPZO'].

References Cited UNITED STATES PATENTS 2,149,658 3/1939 Armstrong313-178 2,556,254' 12/1951 Garne 313178 2,706,691 4/1955 Schaefer 117-332,748,303 5/1956 Thorington 313-25 2,982,668 5/1961 Gunther 117-333,023,337 2/1962 Repsher 117124 3,094,640 6/1963 Gustin 313-25 JAMES W.LAWRENCE, Primary Exrmiiner.

S. SCHLOSSER, R. SEGAL, Assistant Examiners.

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS OUTER JACKETENCLOSING A QUARTZ ARC TUBE HAVING A PAIR OF THERMIONIC ELECTRODESSEALED THEREIN AT OPPOSITE ENDS AND CONTAINING AN IONIZABLE FILLINGCOMPRISING MERCURY, A METAL HALIDE, AND AN INERT STARTING GAS, THEINTERENVELOPE SPACE BETWEEN SAID JACKET AND SAID ARC TUBE BEINGEVACUATED, AND A HEAT REFLECTIVE COATING ON THE ENDS OF SAID ARC TUBEEXTENDING APPROXIMATELY FLUSH WITH THE TIPS OF SAID ELECTRODES ANDCONSISTING ESSENTIALLY OF CALCIUM PYROPHOSPHATE CA2P2O7.